1. Field of the Invention
The present invention relates to the techniques of the electric melting of glass, in which the conductivity of the melted glass is utilized in order to generate, as a result of the Joule effect, the energy necessary for melting the raw materials.
2. Discussion of the Background
In a first type of electric glass-melting furnace, energy is supplied by means of electrodes completely submerged in the mass of melted glass (referred to below as the bath) and arranged vertically on the hearth of the furnace and/or horizontally on the side walls of the furnace, the composition to be melted being introduced from above, so as to maintain a surface layer forming both a permanent reserve of raw materials and protection for the bath against surface heat losses.
Convective movements take place in the bath of melted glass, these being attributable to the changes in density of the glass according to its degree of heating and contributing to transporting the heat towards the surface layer, where fusion is to take place, and in the remaining volume of the bath. These movements are particularly intense along the electrode on account of the temperature gradient existing in relation to the adjacent mass of glass.
In this configuration where the electrodes are completely submerged, the energy exchange surface between the electrode and the bath is distributed over virtually the entire height of the bath. Consequently, the temperature difference of the glass according to the height of the bath is not very pronounced and the convective movements are of very high amplitude, the stream of hot glass rising along the electrode and then running along the layer of materials to be melted, in order to supply the fusion energy to this layer. This circulation results in continual agitation of the bath which makes it possible to homogenize the mass of melted glass in terms of both composition and temperature.
In general terms, furnaces of this type comprise a shaft made of refractory material, which is very deep, usually of the order of at least 1.5 m, and in which a thickness of melted glass of the order of at least 1.2 to 1.4 m is provided, so that the materials which melt below the surface layer have a sufficient dwell time in the bath to reach a homogeneous state of composition and temperature and thereby produce a satisfactory glass.
The complete submergence of the electrodes, which is considered an advantage since it allows a relatively uniform supply of energy throughout the volume of the bath, imposes constraints, however, since the violent agitation movements mentioned above cause the erosion of the hearth, and measures must be taken in order to protect the hearth against this wear which may also affect the electrodes themselves.
More recently, the electric melting technique underwent a major change which involved dipping the electrodes into the bath through the free surface of the latter, instead of causing them to spring up into the bath from the hearth. This made it possible to solve the difficult problems of replacing the worn electrodes and of leaktightness which were associated with the passage of the electrodes through the refractory of the hearth. It was also possible to reduce the wear of the refractories, since the use of dipping electrodes eliminates direct heating in the region of the hearth, the hot zones being localized in an upper part of the melted bath, and it is therefore possible to limit the generation of convection currents in contact with the hearth. This configuration also allowed an increase in the possibilities for adjusting the production parameters. For more details on this technique and its advantages, reference may be made, in particular, to the document FR-A-2,599,734.
The dimensioning of the furnaces was not appreciably changed by the use of dipping electrodes, and a minimum depth has hitherto been recommended for suitably generating the temperature gradient necessary for establishing the desired relatively low temperature in the region of the hearth: in fact, the temperature profile in the bath is such that the temperature is higher in the vicinity of the electrode and decreases relatively slowly in the direction of the hearth. This minimum depth was also considered to be necessary for producing a glass of good quality.